1. Field of the Invention
The present invention relates to a rechargeable electrochemical apparatus using nonaqueous electrolyte, more particularly a nonaqueous electrolyte secondary battery, and a rechargeable negative electrode.
2. Background of the Art
As nonaqueous electrolyte secondary batteries use alkali metals such as lithium and sodium as negative electrodes, there has been to date actively pursued the development of those batteries that use, as positive-electrode active materials, various intercalation compounds and the like including titanium disulfide (TiS.sub.2) and, as electrolytes, organic electrolytes obtained by dissolving lithium perchlorate or the like in an organic solvent such as propylene carbonate. These secondary batteries feature high battery voltages high energy density, owing to the use of alkali metals in the negative electrode.
However, the secondary batteries of this kind have not yet been put to practical use to date. The main reason for this is that the number of times of possible charge-and-discharge is small (charge-and-discharge cycle life is short) and the charge-and-discharge efficiency in the charge-and-discharge cycle is low. This is caused largely by deterioration of the negative electrode. The lithium negative electrode mainly used at present comprises plate-formed metallic lithium press-bonded to a screen-formed current collector formed of nickel or the like. During the discharging stage, metallic lithium is dissolved into the electrolyte as lithium ions. But, in the charging stage, it is difficult to precipitate the lithium into plate form as before discharge. Rather, there occurs such phenomena that dendrite-like (arborescent) lithium is formed, which falls off breaking near the root, or the lithium is precipitated in small-bead (moss-like) form and disconnects itself from the current collector. Consequently, the battery becomes incapable of being charged and discharged. Further, it often occurs that the dendrite-like metallic lithium thus formed penetrates the barrier separating the positive electrode from the negative electrode and comes into contact with the positive electrode, causing short-circuit, which results in the failure of the function of the battery.
Various methods have been tried up to the present to obviate the defects of the negative electrode mentioned above. In general, there are reported methods which comprise altering the material of the negative-electrode current collector to improve its adhesion to the precipitating lithium or methods which comprise adding to the electrolyte an additive for preventing the formation of dendrite-like metal. But these methods are not always effective. Altering the current collector material is effective for lithium precipitating directly onto the curent collector material; but on further continuation of charge (precipitation), lithium precipitates upon the previously precipitated lithium, whereby the effect of the current collector material is lost. The additives are effective in the early stage of charge-and-discharge cycle; but with further repetition of the cycle, most of the additives decompose owing to oxidation-reduction reactions or the like in the battery, thus losing their effectiveness.
More recently, it has been proposed to use an alloy with lithium as the negative electrode. A well known example is lithium-aluminum alloy. In this case, there are disadvantages in that, although a uniform alloy is formed initially, the uniformity disappears on repetition of charge and discharge and, particularly when the proportion of lithium is large, the electrode becomes fine-grained and disintegrates. It has also been proposed to use a solid solution of silver and alkali metal [Japanese patent application kokai (laid-open) No. 73,860/81; U.S. Pat. Nos. 4,316,777 and 4,330,601]. In this case, it is described that no disintegration occurs as with the aluminum alloy; but only a small amount of lithium goes into the alloy at a sufficiently high rate and sometimes metallic lithium precipitates without alloying itself; to avoid this, use of a porous body or the like has been recommended. Accordingly, the charge efficiency is poor; with alloys containing large amount of lithium, the pulverization caused by charge-and-discharge is gradually accelerated, resulting in sharp decrease in cycle life.
Further, there is an idea of using lithium-mercury alloys [Japanese patent application kokai (laid-open) No. 98.978/82] or of using lithium-lead alloy [Japanese patent application kokai (laid-open) No. 141,869/82]. In the case of lithium-mercury alloys, however, the negative electrode changes into mercury metal in the form of liquid droplets as the result of discharge, and cannot maintain the form of the electrode. In the case of lithium-lead alloys, the pulverization of the electrode due to charge-and-discharge is more severe than in silver solid solution.
Further, it is conceivable to use lithium-tin or lithium-tin-lead alloy. But when these alloys are used, the pulverization of the alloy also occurs with the increase of the amount of lithium incorporated into alloy on charging, which makes maintaining the form as an electrode impossible.
Thus, no negative electrode rechargeable in a nonaqueous electrolyte has yet been found that can be satisfactorily used in practice.